Electrolytic production of alkali salts



2,770,589 Patented Nov. 13, 1956 United States atent Ofifice ELECTROLYTIC PRODUCTION OF ALKALI SALTS F IODIC ACID Charles L. Mehltretter, Peoria, 111., assignor to the United States of America as represented by the Secretary of Agriculture No Drawing. Application December 21, 1955, Serial No. 554,595

2 Claims. (Cl. 20495) (Granted under Title 35, U. S. Code (1952), see. 266) A non-exclusive, irrevocable, royalty-free license in the invention herein described, for all governmental purposes, throughout the world, with the power to grant sublicenses for such purposes, is hereby granted to the Government of the United States of America.

This invention relates to and has as its prime object the provision of methods for producing alkali salts of iodic acid by electrolytic oxidation of iodine in an aqueous solution of an alkali capable of furnishing alkali metal or alkaline earth metal ions. Further objects and advantages of the invention will be evident from the following description.

It has previously been shown by Willard and Ralston [Trans Electrochem. Soc., vol. 62, pp. 239-254 (1932)] that iodine suspended in dilute hydrochloric acid can be electrolytically oxidized to iodic acid. In their process, a divided cell is used. The anolyte is a dilute solution of hydrochloric acid containing suspended iodine, "the anode is platinum, the catholyte is nitric acid and the cathode is gold plated copper tubing. This prior process, while it does produce iodic acid, has many tech'no logical disadvantages which are explained as follows: For one thing, the process requires the use of very expensive electrodes, namely a platinum anode and a goldplated cathode because of the corrosive nature of their electrolytes. Another point is that the electrolysis must be extended after the complete oxidation of iodine to iodic acid in order to remove deleterious hydrochloric acid through chlorine formation at the anode. The current efiiciency of the process is thus decreased and corrosive chlorine is liberated. Another point is that the nitric acid catholyte is gradually reduced to ammonia by hydrogen liberated at the cathode and the catholyte must be changed when it becomes alkaline, thus interrupting the electrolysis and causing the use of excessive amounts of nitric acid. Also, nitric acid migrates into the anolyte and as a result, when the iodic acid is removed from the anolyte, evaporation of this solution must be conducted in special non-corrosive, hence expensive, equipment.

It has now been found that iodic acid can be produced in the form of its alkali salts by electrolysis of a solution of iodine in an aqueous alkali capable of furnishing alkali metal or alkaline earth metal ions. in essence the process involves passing an electric current through a divided cell wherein the anolyte is a solution of iodine in an aqueous alkali capable of furnishing alkali metal or alkaline earth metal ions and where the catholyte is an aqueous alkali also capable of furnishing alkali metal or alkaline earth metal ions. The process of this invention yields many technological advantages. In the first place the electrodes can be made of relatively inexpensive materials such as graphite for the anode and steel for the cathode. Also corrosion problems are not encountered so that special non-corrosive equipment is not necessary in the recovery of the alkali iodates. A further point is that the current eificiency of the electrolysis is high,

namely, 92 percent as against 76-88 percent obtained by Willard and Ralston after electrolysis to remove hydrochloric acid as chlorine. An additional advantage is that in the process of this invention the anolyte is an aqueous solution of an alkali capable of furnishing alkali metal or alkaline earth metal ions and iodine is actually soluble therein. As a result oxidation takes place more rapidly than in the prior process wherein the iodine was merely suspended in dilute acid. Previous to this invention it had not been known that iodine could be sub stantially converted to iodate in an aqueous solution of an alkali capable of furnishing alkali metal or alkaline earth metal ions by an electrolytic procedure.

The production of iodates from iodine in accordance with this invention is believed to be a result of the following actions caused by the passage of the current through the electrolytic system. Hydroxyl ions in the anolyte are attracted to the anode where they are discharged with the formation of water and nascent oxygen, the latter of which in turn reacts with the iodine (existing in solution probably as hypoiodite) forming iodate ions. Sodium ions (or other alkali metal ions, depending on the nature of the alkali used) migrate to the cathode 'thus causing a gradual increase in alkalinity of the catholyte and a decrease in aklalinity of the anolyte.

The conditions and materials used in the process of this invention may vary within wide limits. As noted above, both the anolyte and the catholyte are aqueous solutions of an alkali capable of furnishing alkali metal or alkaline earth metal ions iodine being dissolved in the anolyte. The alkali in these solutions is preferably sodium hydroxide but other alkalis capable of furnishing alkali metal or alkaline earth metal ions may be used, for example, potassium hydroxide, calcium hydroxide, barium hydroxide, strontium hydroxide, sodium carbonate, potassium carbonate, or mixtures of these. The concentration of alkali is not critical and generally a solution containing about 2 to percent of the alkali is employed. It is evident that with the alkalis of limited solubility, solutions of the upper concentration range cannot be achieved. To supply the greatest possible amount of iodine for oxidation to iodate, it is preferred to add enough iodine to the solution of alkali to provide an anolyte approximately saturated with iodine. To assist in dissolving a maximum amount of iodine in the anolyte one can begin the process with an anolyte solution which is highly concentrated with respect to alkali and which will thus take up a large proportion of iodine. On the other hand, the catholyte would initially be quite dilute in alkali. As the process proceeds, the alkali metal ions (sodium for example, where sodium hydroxide is the alkali) will migrate to the catholyte. As a result when the process is completed, the anolyte will contain only a small amount of alkali and recovery of the iodate will not be impeded by the presence of an excess of alkali.

During the electrolysis it is preferred to keep the anolyte well stirred and to use a temperature within the range of about 5 to C. The amount of current may vary over a wide range. In general, it has been found that current densities on the order of 0.5 to 12 amperes per square decimeter of anode surface give good results.

The apparatus required for the electrolysis is a 2-compartment cell arrangement well known in the art, the anolyte or catholyte compartments being separated by a porous partition made of aluminium oxide, silicon carhide, or the like. A carbon electrode as the anode is preferred. This element being immersed in the anolyte, operates in an oxidizing and alkaline environment and only moderate attack of the electrode occurs. Other materials may be used which are capable of resisting an alkaline oxidizing environment as well known in the field of electrochemistry. However, carbon is preferred as being relatively inexpensive and giving entirely satisfactory results. In the case of the cathode, this electrode being immersed in the catholyte, is subjected to an alkaline reducing environment and any of the inexpensive, common metals such as iron or its alloys can be used for this element.

After oxidation of the iodine to iodate, the latter may be recovered from the anolyte by various ways. For example, the anolyte may be subjected to evaporative concentration to cause crystallization of the alkali iodate,

alcohol may be added to the anolyte to cause precipitation of the iodate, or combinations of such procedures may be employed. Alternatively, the anolyte may be acidified with sulphuric acid and the resulting solution containing iodic acid may be used as the anolyte for the electrolytic oxidation of starch and other polysaccharides as disclosed in Patent No. 2,713,553.

The invention is further demonstrated by the following examples, it being understood that these examples are furnished only by way of illustration and not limitation.

Example I A 2-compa1'tment cell was made up of a 600 ml. beaker containing an Alundum porous cup as the catholyte chamber and the beaker as the anolyte chamber. The catholyte was 30 ml. of 5 percent sodium hydroxide solution in which was immersed a steel rod cathode. The anode was a graphite plate 15 cm. x 5 cm. x 1 cm. in dimensions and was partly immersed in the anolyte. The effective area of the anode for the electrolysis Was 42 sq. cm. The anolyte was 300 ml. of 7 percent sodium hydroxide solution containing 25 grams of iodine in solution. The anolyte solution was mechanically stirred while passing 5 amperes of current through the cell with occasional addition of 40 percent sodium hydroxide solution to maintain a current of 5 arnperes. After 5.5 hours .a negative test for free iodine was obtained. Analysis showed that 36.4 grams of sodium iodate were present and 0.8 gram of sodium periodate. The yield of iodate was thus 93 percent of theory and of the mixture of iodate and periodate 95 percent of theory. The current efliciency of the cell was 92 percent.

The solution was filtered from carbon particles and adjusted to pH 2 with sulfuric acid. It could be used directly for the electrolytic oxidation of starch to oxystarch as divulged in U. S. Patent 2,713,553.

Example II Using the same setup and conditions as in the previous example, the final alkaline anolyte solution after filtration from carbon particles contained 37.7 gm. of sodium iodate (97 percent of theory) and no sodium periodate. The alkaline solution was concentrated on the steam bath to crystallization, cooled, filtered, and the crystalline product washed free of alkali with aqueous alcohol. The White product after drying at 100 weighed 22.9 grams. It analyzed 100 percent sodium iodate. The filtrate was concentrated to about ml. and was treated with 200 ml. of methanol to precipitate residual sodium iodate. The white product was filtered, washed with methanol and dried at 100 C. It weighed 14.6 grams and analyzed percent sodium iodate. Total recovery of crystalline sodium iodate was 94 percent of theory.

Having thus disclosed my invention, I claim:

1. A process for preparing an alkali iodate comprising passing an electric current through a 2-compartment cell divided by a porous partition, said cell containing in one of its compartments as anolyte a solution of elemental iodine in an aqueous alkali capable of furnishing ions selected from the group consisting of alkali metal ions and alkaline earth metal ions in which is immersed an anode capable of resisting an alkaline oxidizing environment and containing in its other compartment as catholyte an aqueous solution of an alkali capable of furnishing ions selected from the group consisting of alkali metal ions and alkaline earth metal ions in which is immersed a cathode capable of resisting an alkaline reducing environment, whereby the elemental iodine is oxidized to the corresponding alkali iodate at the anode.

2. A process for preparing sodium iodate comprising passing an electric current through a 2-compartment cell divided by a porous partition, said cell containing in one of its compartments as anolyte a solution of elemental iodine in aqueous sodium hydroxide in which is immersed a carbon anode and containing in its other compartment as catholyte an aqueous solution of sodium hydroxide in which is immersed an iron cathode, whereby the elemental iodine is oxidized to sodium iodate at the said carbon anode.

References Cited in the file of this patent Trans. Electrochemical Soc., vol. 62, pp. 239254 (1932), Willard et a1. 

2. A PROCESS FOR PREPARING SODIUM IODATE COMPRISING PASSAGE AN ELECTRIC CURRENT THROUGH A 2-COMPARTMENT CELL DIVIDED BY A POROUS PARTITION, SAID CELL CONTAINING IN ONE OF ITS COMPARTMENTS AS ANOLYTE A SOLUTION OF ELEMENTAL IODINE IN AQUEOUS SODIUM HYDROXIDE IN WHICH IS IMMERSED A CARBON ANODE AND CONTAINING IN ITS OTHER COMPARTMENT AS CATHOLYTE AN AQUEOUS SOLUTION OF SODIUM HYDROXIDE IN WHICH IS IMMERSED AN IRON CATHODE, WHEREBY THE ELEMENTAL IODINE IS OXIDIZED TO SODIUM IODATE AT THE SAID CARBON ANODE. 